Added twi.c which simplifies working with TWI(i2c)

The rtc now works and we can query it for a new timestamp
Removed debugging printf statements
Made the current flag read and writes more clear
Removed uncommented print statements
Merge branch 'twi_support' of github.com:UnicornRaceEngineering/g5-nodes into twi_support

Conflicts:
	examples/twi_master/m41t81s_rtc.c
	examples/twi_master/m41t81s_rtc.h
	examples/twi_master/main.c
	libat90/twi.c
	libat90/twi.h